The layer of the crust that averages 25 miles in thickness is the continental crust.

Here's why:

* Continental crust is thicker and less dense than oceanic crust. It's composed primarily of granite and other felsic rocks.

* Oceanic crust is thinner and denser, made up mostly of basalt and other mafic rocks. It averages about 4 miles thick.

While the exact thickness of the continental crust can vary, 25 miles is a good average.